Can Thermotunneling Improve the Currently Realized Thermoelectric Conversion Efficiency?

Abstract

In a tunnel junction, electrons can overcome a nanoscale vacuum gap after the application of an electrical voltage. A temperature difference rather than an electrical voltage applied at the junction gives rise to an analogous thermoelectric tunnel effect called thermotunneling. This effect opens the possibility of thermoelectric conversion without phononic thermal backflow, which has encouraged optimism regarding the potential of thermotunneling for power generation and refrigeration. However, thermotunneling implies a photonic thermal backflow caused by radiative heat exchange amplified by photon tunneling of evanescent modes. An investigation based on a free electron model and comprising the combined influence of both electronic and photonic heat transfer through a vacuum tunnel gap is presented. An upper limit M = π2/12 on the dimensionless thermoelectric figure of merit practically attainable by thermotunneling can be stated. This means that thermo- tunneling cannot outperform the maximum M values achieved by thermoelectric materials research to date.